Offset printing apparatus incorporating a removable cassette comprise a fixed frame on which are mounted, in particular, an inking device and a dampening device which ensure deposit of a film of ink and water on one or more inking rollers with a peripheral layer of supple material. The ink roller or rollers are themselves mounted to rotate on the fixed frame of the apparatus. Furthermore, such an apparatus uses a removable cassette essentially comprising a chassis on which are rotatably mounted two cylinders of parallel axes, of the same diameter, rotating in opposite directions at the same peripheral speed, namely a plate cylinder and a blanket cylinder, made of supple material, which transfers the print on a web having to be printed. The removable cassette is introduced and housed in the fixed frame so that, when it is completely engaged in this frame, the plate cylinder, i.e. the one which bears the plate used for the print, is in contact with the ink roller or rollers, so as to be able to receive therefrom the ink and the dampening water which are necessary for ensuring offset printing.
Such an offset printing apparatus with removable cassette offers the advantage that it lends itself very well to relatively short series of printing, since it is very easy, when it is necessary to pass from one printing series to another, to replace the cassette used for the preceding printing series by a new cassette appropriate for printing the new series. However, these apparatus pose a problem when one cassette is to be replaced by another, namely that it is necessary, upon each change of cassette, to proceed with a fresh adjustment of the "touch" of the inking rollers on the new plate cylinder placed in position. This "touch", which is necessary in order to obtain a print of good quality, corresponds to the crushing that the supple peripheral layer of each inking roller undergoes in its zone of contact with the plate cylinder, as the two cylinders are applied against one another under a certain pressure. Now, this touch may vary in time for several reasons. In the first place, the diameter of each inking roller may vary in time, due to wear thereof, and such variation in the diameter therefore influences the touch. Secondly, the overall outer diameter of the plate cylinders of different cassettes are slightly different, although the bodies of these cylinders have the same diameter. In fact, a covering of variable thickness is always disposed between each plate and the body of the cylinder, which brings about a slight variation of the outer diameter of the cylinder equipped with its plate. This variation of the outer diameter of the plate cylinder in turn influences the touch between each inking roller and the plate cylinder after the cassette has been placed in position.
It is an object of the invention automatically to obtain, by simple means, after a new cassette has been placed in position, the desired value of the touch between each inking roller and the new plate cylinder pressed thereagainst.